I had a bit of a disaster on this site recently. On Saturday I went about upgrading Drupal from version 5 to 6. I have always had no problem doing upgrades and fully expected this one to go fine as well.
I turned off all the extra modules, changed the site theme to the default, then pointed my domain to another Drupal directory I had with Drupal 6 in it. The actual upgrade was smooth, but I ran into a snag with the image module.
I then backed out of the upgrade. I pointed my domain back at the Drupal 5 install then did a DB restore. This is where it went bad.
The DB restore I chose said it was from 9 hays ago but I had read 9 hours ago. I waited for the restore to complete but I panicked when it seemed to take a while. I started another restore from the next oldest restore point, only to find my database completely empty. It was late at night on a weekend. I made a basic html page for my site and entered a support request.
The next day as I was preparing to head up island for Thanksgiving I checked my DB and saw that it had been restored. Loading my site I found that everything was there. Sure enough, Dreamhost support had come through and restored my DB.
Turns out the restore I had originally attempted was named incorrectly. I still don’t know why it didn’t work when I tried it.
The lesson I learned: back up the DB first! The dreamhost wiki has some information regarding this. I’ve instituted the nightly db backups for almost all my databases. With the script, I can run it just before an upgrade so I have a convenient way to revert.